Effects Of Wind And Gravity

Marksmanship instructors-trainers should know how the effects of wind and gravity influence the flight of the bullet, and soldiers should know how to compensate for such bullet displacement. This instruction is appropriate for all marksmanship training and concurrent training.

a. Effects of Gravity. Gases created by gunpowder push each round out the end of the barrel. The barrel must be elevated slightly to allow the round to travel farther, creating an arc. The round will travel straight until it slows down and is gradually pulled down to the ground by gravity. Each round fired will be pushed approximately the same distance and will roughly follow the same path. When the firer zeroes his weapon he is aligning his line of sight to cross the path of the round at the distance he wants to zero his weapon. For example, a 300-meter zero means that the line of sight will cross the path of the round at 300 meters. If the firer is going to engage a target at a distance other than 300 meters (excluding 25 meters) the path of the round will hit the target either before or after it crosses the line of sight. If the firer wants his rounds to impact center of mass he will have to adjust his aiming point up or down to account for gravity. The farther the round travels the faster it begins to fall.

b. Adjusted Aiming Point Based on Gravity. An adjusted aiming point (Figure 5-26) is intended to increase hit probability when properly presented. However, soldiers can be easily confused, which could result in degraded performance. All soldiers should be taught to aim center mass unless they are confident they know the range to the target. If adjusting the point of aim confuses the soldier he should aim center mass of the target. Using these aiming points places the center of each shot group center mass of the target (assuming a perfect zero and no shooter error). These adjustments are small and should only be applied by competent firers who wish to improve their firing performance. Because the difference between the M16-/M4-series weapons is so small, and to avoid confusion, the same adjusted aiming points should be used regardless of the weapon being fired.

Bullet Wind Effect Constants
Figure 5-26. M16-/M4-series weapons aiming points.

c. Effects of Wind. Wind affects the bullet similar to the way gravity does: the farther the round travels the farther the wind will push the round in the direction the wind is blowing. The faster the wind is blowing the farther the wind will push the bullet.

(1) Wind Direction. The effects of wind vary depending on changes in wind speed and direction. Wind is classified by the direction it is blowing in relationship to the firer-target line. The clock system is used to indicate wind direction and value (Figure 5-27, page 5-28).

(a) Winds that blow from the left (9 o'clock) or right (3 o'clock) are called full-value winds, because they have the most effect on the bullet.

(b) Winds that blow at an angle from the front or rear are called half-value winds, because they have about one-half the effect on the bullet as full value winds.

(c) Winds that blow straight into the firer's face or winds that blow straight into the target are termed no-value winds, because their effect on the bullet is too small to be concerned with.

Wind Effects Cal
Figure 5-27. Determine wind value using the clock method.

(2) Wind Speed. Wind is highly variable and sometimes quite different at the firing position than at the target position. Even though the wind is blowing hard at the firing line, trees, brush, or terrain could protect the bullet path. The wind can vary by several miles per hour between the time a measurement is taken and when the bullet is fired. Therefore, training time should not be wasted trying to teach soldiers an exact way to measure wind speed. Soldiers should understand that the wind can blow the bullet off course but they should not overcompensate and miss targets because of applying too much hold-off. A wind gauge can be used for precise measurement of wind velocity. When a gauge is not available, velocity is estimated by one of the following methods.

(a) Flag Method. If the firer can observe a flag or any cloth-like material hanging from a pole, he should be able to estimate the angle formed at the juncture of the flag and pole. As shown in Figure 5-28, dividing this angle by the constant number 4 equals the wind velocity in miles per hour.

Effect Wind Speed Evaporation
Figure 5-28. Determine wind speed using the flag method.

(b) Pointing Method. If a flag is not visible, a piece of paper, leaf or other light material can be dropped from the shoulder. By pointing directly at the spot where it lands, the angle can be estimated. As shown in Figure 5-29, dividing this angle by the number 4 determines the approximate wind speed in miles per hour. This indicates conditions at the firing position, which could be different at the target position.

Wind Value Rifle
Figure 5-29. Determine wind speed using the pointing method.

(c) Observation Method. If the flag or pointing methods cannot be used, the following information can assist in determining wind velocities:

• Winds less than 3 miles per hour can barely be felt by the firer, but the presence of slight wind can be determined by drifting smoke.

• Winds of 3 to 5 miles per hour can be felt lightly over the firer's face.

• Winds of 5 to 8 miles per hour constantly move the leaves of trees.

• Winds of 8 to 12 miles per hour raise dust and loose paper.

• Winds of 12 to 15 miles per hour cause small trees to sway.

d. Adjusted Aiming Point Based on Wind Speed. Figure 5-30 illustrates how the effects of wind on the bullet are similar to the effects of gravity—as range increases, the effect of wind increases. A 10-mile-per-hour full-value wind moves an M16A1 (M193) bullet from about 1/2 of an inch at 25 meters to about 15 inches at 300 meters. (Using the data presented in Table 5-3, wind effects for all conditions can be determined.) A wind of greater speed increases bullet movement by a uniform amount—a 15-mile-per-hour wind moves the bullet 3/4 of an inch at 25 meters and about 22.5 inches at 300 meters. A half-value wind would move the strike of the round in a 10-mile-per-hour wind 1/4 of an inch at 25 meters and 7.5 inches at 300 meters. (This chart can be used for M855 ammunition, also.)

Round Magazine For Rifle

RANGE

WIND

(in meters)

SPEED

25

50

75

100

150

175

200

250

300

(mph)

DISTANCE MOVED (in inches)

5

1/4

3/8

1/2

1

2

2.5

3.5

5

7.5

10

1/2

3/4

1

2

4

5

7

10

15

15

3/4

1-1/8

1.5

3

6

7.5

10.5

15

22.5

Table 5-3. Calculated adjusted aiming point based on wind speed (full value).

Table 5-3. Calculated adjusted aiming point based on wind speed (full value).

e. Adjusted Aiming Point Based on Gravity and Wind Speed (Figure 5-31). Wind has a minor effect on the M16 bullet relative to the size of the target at ranges out to 100 meters. When engaging targets in excess of 150 meters in heavy winds, adjusting the aiming point for the wind increases the probability of a hit. Wind effects are uniform in relation to speed—that is, a 5-mile-per-hour wind has one-half the effect of a 10-mile-per-hour wind, and a 20-mile-per-hour wind has twice the effect of a 10-mile-per-hour wind.

(a) Firers must adjust their aiming point into the wind to compensate for the effects of wind. If they miss a distant target and the wind is blowing from the right, they should aim to the right for the next shot. A guide for the initial adjustment is to split the front sight post on the edge of the target facing the wind.

(b) The newly assigned soldier should aim at center mass for the first shot, then adjust for wind when he is confident the wind caused the target miss. Experienced firers should be able to apply the appropriate hold-off for the first shot, but the basic rule must be followed— when in doubt, aim at center mass.

M16 Front Sight And Silhouette
Figure 5-31. M16-/M4-series weapons adjusted aiming point based on wind speed.

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